Polluting emissions from internal combustion engines are increasingly subject to regulation. These regulations have led to the use of a wide variety of emissions control technologies.
One approach to reducing regulated emissions is selective catalytic reduction (SCR). SCR is typically used to reduce oxides of nitrogen (NOx) emissions in lean burn engine exhaust, such as diesel exhaust. SCR methods mix a reductant with the engine exhaust, and flow this mixture through a special catalyst. The reductant sets off a chemical reaction within the catalyst that converts NOx in the exhaust into nitrogen, a natural component of air.
For SCR applications that are not necessarily automotive, several reductants are currently used. These reductants include anhydrous ammonia, aqueous ammonia or urea. Pure anhydrous ammonia is toxic and difficult to safely store, but needs no further conversion to operate within an SCR catalyst. It is typically favored by large industrial SCR applications. Aqueous ammonia must be dehydrated in order to be used, but it is safer to store and transport than anhydrous ammonia. Urea is the safest to store, but requires conversion to ammonia through thermal decomposition and hydrolysis before use as a reductant.
For automotive SCR applications, a solution of automotive-grade urea is typically used as the reductant source. For this application, the urea solution is sometimes referred to as diesel exhaust fluid (DEF) or AdBlue in Europe.
In SCR emissions aftertreatment systems, the urea water solution decomposition process is complex, and presents issues with solid deposit formation. These deposits can affect the efficiency of urea decomposition, and if large enough, can inhibit exhaust flow. Deposit formation is a significant challenge to SCR aftertreatment system designers due to the complexity of contributing factors such as temperature, flow rate, flow distribution, dosing rate, wall or mixer surfaces, and droplet size distribution.